Ad limina apostolorum (Latin, meaning "to the treshold of the Apostles") was originally, in the Middle Ages, a reference to a pilgrimage to the tombs of the Apostles Peter and Paul in Rome.
In modern times, it usually refers to the visits to Rome (to the tombs of the Apostles, and to the Pope) that all Catholic bishops are required to make at least every five years. The visit includes prayer at the tombs of Peter and Paul at respectively San Pietro in Vaticano and San Paolo fuori le Mura.